Various types of energy sources, consisting of photocells activated by some type of nuclear radiation, are known in the prior art. These devices, sometimes referred to as "nuclear batteries" or "atomic batteries," generally convert nuclear electromagnetic radiation into electrical energy by one of two methods, namely, single conversion or double conversion. Single conversion nuclear batteries generate electrical energy by converting the nuclear radiation (i.e. alpha particles, beta particles, gamma radiation or x-rays) into electrical energy by direct absorption of the nuclear radition at the p-n junction of a semiconductor material. U.S. Pat. Nos. 3,094,634, 3,304,445 and 4,024,420 are exemplary.
Double conversion nuclear batteries generate electrical energy by converting the nuclear radiation into electromagnetic radiation, usually by irradiating a phosphorescent material that will generate light in the visible spectrum, and then converting that electromagnetic radiation into electrical energy by absorbtion of the electromagnetic radiation at the p-n junction of a semiconductor material, usually a typical photovoltaic cell. U.S. Pat. Nos. 3,031,519, 3,053,927, and 3,483,040 are exemplary.
Although a variety of self-luminous, low light sources have been available for a long time (e.g. radium- and tritium-activated phosphors used for creating self-luminous paints for watch dials, etc., U.S. Pat. Nos. 3,033,797, 3,325,420 and 3,342,743), it has generally been regarded that such materials were unsuitable for commercial use for the conversion of light into electricity. The low levels of radioactivity associated with such materials, though generally not harmful or dangerous, do not provide an adequate source of power for the nuclear batteries of the type known in the prior art. In addition to the low light level (50 micro-lamberts or less), such sources may also be characterized by rapid and unpredictable light decay and, in the case of radium-activated light sources, may produce undesirable radiation hazards associated with their decay products.
Further, even if the decay is slow and predictable, decay nevertheless exists, which causes the level of illumination to diminish with the passage of time. This diminution causes the output voltage of the battery to diminish as time progresses.
Though the concept of a long-life, electrical energy source activated by a radioactive material is attractive and has many potential applications, none of the prior art devices have been able to create a safe, yet sufficiently powerful, energy source that is commercially feasible. Accordingly, there is a continuing need to develop a safe, practical, long-lived, electrical energy source of constant voltage which is powered by a radioactive source.